A MIMO (Multiple Input, multiple output) wireless communication system (see FIG. 1) is one which comprises a plurality of antennas at the transmitter and one or more antennas at the receiver. The antennas are employed in a multi-path rich environment such that due to the presence of various scattering objects (buildings, cars, hills, etc.) in the environment, each signal experiences multipath propagation. Thus there are numerous scattered signals between the transmit and receive antennas. User data is transmitted from the transmit antennas using a MIMO transmission method, for example space-time coding (STC) or BLAST as is known in the art, typically with many channels separated by frequency, time slots, or coding. The receive antennas capture the transmitted signals and a signal processing technique is then applied as known in the art, to separate the transmitted signals and recover the user data. FIG. 1 shows a base station BS1 having two or more spatially separated antennas, transmitting to corresponding antennas on user equipment UE1. It is not untypical for one of the MIMO channels to have much better reception than the other, despite the close proximity of the respective antennas.
MIMO wireless communication systems are advantageous in that they enable the capacity of the wireless link between the transmitter and receiver to be improved compared with previous systems in the respect that higher data rates can be obtained. The multipath rich environment enables multiple channels (these are what are referred to as sub-channels in the remainder of the document) to be transmitted between the transmitter and receiver, and distinguished at the receiver only by the spatial characteristics, even though the same frequency, code or time slot is used. Even line of sight signals can potentially be separated into MIMO sub channels based on spatial characteristics. Data for a single user can then be transmitted over several paths in the air by inverse multiplexing the data into several streams. These are transmitted simultaneously using the same frequency or time slots or codes, and remultiplexed at the receiver. Consequently, higher spectral efficiencies are achieved than with non-MIMO systems.
Also, as the multipath characteristic varies with time, especially for mobile users, adaptive modulation coding (AMC) can be used to achieve higher data rates where the multipath and interference and noise allow. Conventional hand off techniques are used in cell based MIMO systems to hand off all streams of a MIMO channel to a neighboring base station. The trigger for such a “hard” hand off is usually a signal strength indication of the downlink, measured at the user equipment.
US Patent Application 20030003863 shows link adaptation for MIMO transmission schemes. Information to be transmitted is divided into a plurality of subsignals (defined as the signal carried on a subchannel). In the receiver the different receive signals are processed so that subsignals are detected and decoded and the contribution of each detected and decoded subsignal is subtracted from the receive signals. A feedback channel from receiver to transmitter is used to send control information to the transmitter to optimize the usage of the MIMO channel. In the receiver, the link quality of each subsignal is determined and is transmitted to the receiver via the feedback channel. In the transmitter, the link quality information can be used to vary the data rate of each subsignal, vary the transmit power of each subsignal, vary the modulation scheme of each subsignal, vary the coding scheme of each subsignal or vary any combination of these properties. Furthermore, the link quality determination may be based on an error rate measurement, a noise ratio measurement, or a capacity measurement The link quality measurement may be fast-adaptive, e.g. when it is based on the instantaneous calculated capacities of each subsignal. Fast means that the measurement period is shorter or substantially equal to the time period in which fast fading becomes relevant. As fast fading is dependant on the Doppler shift of the signal, the time period is also dependent on the velocity a receiver moves relative to the transmitter. For slow-adaptive embodiments a capacity calculation based on an average of the calculated capacities of each layer with respect to a longer time period or a capacity calculation that takes the outage of the calculated capacities of each layer with respect to a longer time period, may be applied.
One limitation with existing MIMO systems concerns the large size of the transmit and receive antenna arrays. Another limitation with existing MIMO systems is that they are designed for use in environments where scattering occurs rather than for line of sight situations. More significant in many cases are the following limitations:    a) The coverage of MIMO systems can be very uneven. MIMO provides much improved data rates for users with good C/I, typically near the base station, but provides little improvement to users in a poor C/I situation (typically at cell edges) so their data rate remains low.    b) The ‘sub-channels’ of a MIMO system often have very uneven capacity. This is dependent on the propagation characteristics of the channel and can be particularly uneven in a propagation environment where one multipath component is very dominant. These poor quality sub-channels are used to provide a very small additional data rate to the user which is wasteful.